Structural members used to reinforce a vehicle are required to have high strength and light weight. Further, structural members are often required to have impact resistance and exhibit a reduction in the amount of deformation upon an impact. In view of this, one structural member sometimes includes both impact-resistant portions and energy-absorbing portions. To achieve this, the strength distribution in a structural member may include various portions with different strengths.
For example, WO 2013/137454 (Patent Document 1) discloses a hot-pressed molded product that strikes a balance between high strength and the amount of extension for each of its portions, wherein a single molded product is provided with regions representing impact-resistant portions and energy-absorbing portions without application of welding.
JP 2011-173166A (Patent Document 2) discloses a complex press-working apparatus that facilitates creation of separate high-strength portions and low-strength portions within a single press-molded product.
Japanese Patent No. 5894081 discloses a B-pillar for a vehicle. This B-pillar includes a hat-shaped section including a central flange, two web portions, and two side flanges. A hat-shaped section is hot-formed from a flat plate of boron steel and is then hardened. The hat-shaped section has a fracture strength above 1400 MPa. During hardening, some portions of the side flanges are not completely hardened and have a fracture strength below 1100 MPa.